This invention relates to molding compositions and articles made therefrom. More particularly, this invention relates to molding compositions comprising a thermoplastic resin and graphite. Such compositions may be suitably molded into acoustic articles such as diaphragms, arm pipes, head shells, cantilevers or the like. This invention also relates to such moldings.
For the fabrication of elements used in vibration systems in acoustic instruments, for example, diaphragms, head shells, tone arms, cantilevers and the like, materials are required having such properties as light weight, high stiffness and high elasticity. Addition of high internal loss to this list of properties is desirable. In the case of speaker diaphragms, light weight and increased stiffness enable the diaphragms to reproduce sound up to a higher frequency range without partial vibration. Internal loss should be high enough to prevent the rapid rise of sound pressure around resonance frequencies and to improve damping characteristics. In the cases of vibration system elements, for example, head shells, tone arms, cantilevers or the like, it is required to reduce the equivalent mass at a stylus tip as low as possible. This means that materials should have light weight, high stiffness and high elasticity to ensure a sufficient mechanical strength at a reduced wall thickness. Internal loss should also be high enough to accomodate partial vibration.
Known light weight, high elasticity materials are metals such as aluminum, titanium and beryllium, and composite materials such as carbon fiber reinforced plastics and graphite in synthetic resins. Particularly, these metals have been used to mold diaphragms and head shells but they are very poor in internal loss. Materials known of their high internal loss are paper, synthetic resins and their composite materials. However, these materials have a low elasticity and hence, a low specific modulus of elasticity E/.rho.. There is a need for light weight, high elasticity, high internal loss materials.
The inventors previously proposed diaphragms, arm pipes and head shells made from a kneaded mixture of flaky graphite and a high molecular compound. Articles molded from such compositions, surface oxidized articles and carbonized articles are disclosed in U.S. Ser. Nos. 968,912 (filed Dec. 13, 1978), 35,425 (filed June 29, 1979), 63,531 (filed Aug. 3, 1979), 53,532 (filed Aug. 3, 1979), and 78,045 (filed Sept. 24, 1979). These materials are relatively light weight and have a high modulus of elasticity and a high internal loss. Diaphragms, arm pipes and head shells made therefrom show good properties for their purposes.